Stagecoach must break up its hop-on, hop-off tourist bus business in New York or sell assets because it has a monopoly, according to US regulators.

Stagecoach reacted angrily to the ruling by New York’s Attorney-General and Department of Justice, saying yesterday that it would “robustly defend” its business.

The decision is the latest move in a dispute between Stagecoach and US regulators over Twin America, a joint venture between Stagecoach and another bus operator, City Sights. Twin America, which is majority owned by Stagecoach, was formed in 2009. The US lawsuit alleges that the venture has given Twin America 99